The present invention relates to a method and means for testing transmissive optical fibers, and more to measuring the length of a fiber.
Optical fiber is increasingly being employed for various forms of signal transmission. The length of a fiber affects the degree to which various parameters come into play in changing a transmitted signal. Such parameters include attenuation per unit length and dispersion of different wavelength components of an optical signal.
The total length of the fiber will be proportional to the delay T produced in the fiber. This delay may be expressed by: EQU T=n(.lambda.)L/c, (1)
where T=delay, n(.lambda.) is the refractive index of the fiber, L is fiber length, and c is velocity of light in vacuo. The value of n(.lambda.) varies with wavelength. This value can be determined for a selected wavelength transmission.
The dispersion phenomenon requires the measurement of difference in arrival times of currently launched components of different wavelengths. A system for measuring fiber optic dispersion is disclosed in United Kingdom Patent 2,183,823 and corresponding U.S. Pat. No. 4,770,833 to Roger S. Jones, the assignee therein having the same parent company as the assignee herein. For a fiber length measurement, a fixed wavelength may be chosen for which the value of n(.lambda.) is known or may be derived for a test fiber. The value of n(.lambda.) maybe slightly sensitive to temperature or stress on the fiber, but not to a degree significant for purposes of the presented discussion . Typical values of n(.lambda.) are in the range of 1.4 to 1.6 for silica based telecommunications grade fiber. With n(.lambda.) and c being constant, measurement of fiber delay T maybe used as a measurement for L.
One way to measure T is by measurement of phase shift produced by transmission by the optical fiber of modulated light. By virtue of the delay T, the modulation waveform on the received light is phase shifted by a phase .phi. from the original input waveform. This phase may be expressed as EQU .phi.=T2.pi.f, (2)
Where f is a modulation frequency. Also, EQU .phi.=n(.lambda.)2.pi.f L/c. (3)
The value of .phi. can be determined by the use of a phase sensitive detector.
Static phase shift measurement is subject to undesirable uncertainties. The phase meter (time detector) output is used as a direct current value. Measurement of T is, therefore, subject to customary sources of error such as drift from many sources. Also, the measurement is subject to 1/f noise. Also, this technique is substantially limited to laboratory utilization. The source and the detector must be co-located to achieve synchronization of light source modulation and of phase detector systems.
A differential method eliminates static measurement errors. The present invention utilizes the phenomenon that if F is varied, the phase .phi. will vary. It will be seen that EQU D.phi.=.phi..sub.1 -.phi..sub.2 =2.pi.(.lambda.) (F1-F2)L/c(4)
Where .phi..sub.1 and .phi..sub.2 are phase shifts obtained for signals modulated at frequencies F1 and F2 respectively.